Admin: Global statistics thwarted by weightless monthly log

If a user created one or more non-void monthly logs with no weight entries (for example, containing only exercise rung and/or comment fields), the administrator global statistics report would fail with a division by zero when it attempted to compute the ``coverage'' of the time period by weight log entries. I modified \verb+receive_aggregated_statistics_records+ to ignore returned records with undefined weight fields, as only such records are relevant to the global statistics.